What you're seeing is galvanic corrosion. You can try using a non-conductive grease, but I would go straight for hard-anodizing the aluminum. If the aluminum piece is too large to make anodizing practical, can you redesign to use a bushing?

Anodizing the base Aluminum will help to avoid your problems and will give a longer life for the base. Alternativealy you can use a PVC ( avilable with variable strengths) bush by drilling a hole in the base metal snug fit it, and use the SS304 pin for sliding. If PVC not suitable you can use metalic bushes also.

You should always prevent ss directly onto unprotected aluminium. We even have our stainless fasteners chemy blacked to prevent galv corrosion.

The only problem with hard anodising is that it fretts really easily and things get stuck as by sheer magic. It is almost as if the surface gets pitted which sucks a vacuum between tightly fitting parts. We shear of several bolts every month when we need to reset something. Pain in the backside but we have to live with it.

Take a look at www.tiodize.com at there Hardtuf coatings for aluminum.

Another neat trick you might try is to mechanical zinc plate the 304 SST pin. It may be galvanic but it's more likely fretting corrosion. The sliding action is also work-hardening the aluminum. The mechanical zinc plating is also the most effective means of preventing galling and seizure between threaded SST fasteners that I have found over the years. Mechanical zinc on the nut threads makes it virtually impossible for galling and seizure (cold-welding) to occur.

Iforgot to mention, foolishly, that we produce microscope stages and parts. In the optical world they want everything black as it does not interfere with the light, thus preventing stray reflection or refraction.

Any galv or zinc plating is therefor out of the question. You could double process all the parts but that would make it a very costly excercise. This is why I said we had to learn to live with it.

Thanks for the tips though, much appreciated.

That being the case, Moto has a good suggestion which I can amplify a bit. First, the optical application suggests the problem cannot be galvanic because galvanic corrosion requires an electrolyte in order to form a galvanic cell. Thus, it is probably fretting and the solution lies in reducing the friction coefficient between the two materials.

One way to do that would be to coat the SST pin with black Teflon or similar material. The pin is probably easier to coat than the mating aluminum surface and the coating can be applied and baked in a variety of processes. Depending upon the geometry of the aluminum housing around the hole, it might also be possible to accomplish the same thing by sealing the aluminum circumference with a similar slippery material.

Use steel or plastic coated steel or sprayed on polymer.

Aluminum will never stop corroding or galling of steel fasteners.

Thin coatings get scratched easily and are more costly to repair on site.

Unless you have a hermetic environment, most affordable coatings will flake and peal over time, anyways.

Not to mention the fact a nail will go right through standard aluminum siding in a storm, let alone a 2x4 at hurricane speed. if you want to add more flamables, keep backing it with plywood or worse yet wafer board. No flame problems there, right?

Aluminum would have to be gaged quite thick to be used structurally; with any real load bearing capabilities. It's certainly not cheaper, at such quality or gage, so why must you force such unreliable, problematic siding on the world, in the first place?

Let me guess,

1) you like frustration or...

2) Your in the industrial coatings business and like using difficult process to prove you can build with outdated technologies, right?

I personally love it when people resist change just to wast money and keep control over every single cent of value or profit, while reducing wages and benefits.

Don't you change a thing there buddy! It's supposed to be ridiculous.